Tech. Motor Selection & Gearbox Matching. Inside. + More

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1 Digital Ebook A Design World Resource Go Motor Selection & Gearbox Matching Inside 8 Steps to Selecting the Right Servo 2 Case Study: Heavy Automotive 5 Quiet Gearmotors - Engineering Tips 6 Gearmotors: Achieving the Perfect Motor & Gearbox Match 7 The Basics of Motor Selection 9 + More

2 Page 2 8 Easy Steps to Selecting the Right Servo by Miles Budimir, Motion Control Editor Design World Servo systems provide amazing levels of speed, accuracy, and flexibility in automated equipment when the correct servo is chosen for the application. Unfortunately, choosing the wrong servo can lead to difficulty in tuning, poor accuracy, or underwhelming performance. With so many different servos to choose from, how do you ensure that the right one was chosen? Fortunately, the process of choosing the right servo motor and drive, known as sizing, can be broken down into eight simple steps. Voltage The first and easiest factor to consider is the available power for the equipment. Servos are available in 100 VAC, 200 VAC, and 400 VAC models, and are compatible with single phase or three-phase power. Motion Profile For equipment that performs a repetitive operation, plot out the required motor speeds throughout the cycle. Be sure to allow for acceleration and deceleration time; servomotors are not magic, and cannot make step changes in speed. For non-repetitive operations such as milling, calculate the peak speed and acceleration required for the application. Torque Torque is how much muscle it takes to rotate a mechanism, and comes from three different sources: accelerating the mechanism s inertia, friction, and external forces such as pressing against an object or gravity. This is the most difficult part to calculate accurately, but is also the most forgiving part of the selection process. Calculate the inertia of each component of the system and add the values. The formulas for calculating rotational inertia of various shapes are readily available on the Internet. Multiply the acceleration by the load inertia to calculate the load s acceleration torque. Calculate friction forces for sliding loads, gravitational forces for vertical loads, and any external forces. Multiply each force by the radius it is acting on (known as the moment ) to calculate the torque. Calculate the peak torque by adding up all the torque values in the worst-case scenario. This is typically when the fastest acceleration is occurring or when there is the most mass on the machine. Add up the torque values from external forces, gravity and friction to calculate the continuous torque requirement. Ideally, the continuous torque requirement would use a root-mean-squared (RMS) calculation, but this is tedious without the help of a software tool. Inertia Ratio Calculating an Inertia Ratio is often overlooked by newcomers to servo sizing, but is arguably the most important factor in determining the performance of a servo system. Inertia Ratio is the ratio of the load s inertia divided by the motor s rotor inertia divided by the square of the gear reduction. To use a boxing analogy, if torque is how strong the fighter is, inertia ratio is the weight class. Not only does the servo system need to have enough torque to move the load, it must have the ability to accurately control the load. In an ideal world, the mechanism would be mechanically rigid and torque from the motor would be transferred smoothly and without delay to the load. In the real world, couplings flex, belts and chains stretch, and gears have backlash. These imperfections can be minimized but not completely eliminated. When the motor begins to move, the machine winds up like a spring, and begins to push back on the motor with some tiny delay. This spring effect is magnified with large inertia ratios. Servo systems use the feedback from the built-in encoder and the PID algorithm in the amplifier to accurately position the motor, but this tiny spring delay can cause oscillations and loss of control if the loop gains are too high. Reducing the gain will stop the oscillation, but at the cost of responsiveness. While basic servo drives may require inertia ratios of 3:1 or smaller, high performance servo drives have auto-tuning, vibration suppression, resonance filters, and disturbance compensation functions that allow up to a 30:1 inertia ratio without sacrificing performance. A ratio of 1:1 will give excellent performance, but usually results in an oversized motor. Ratios less than 1:1 waste power with no performance advantage. Choose a Motor (Round 1) At this point, the key criteria for choosing a servo motor have been defined and it is time to browse the product selection guide to find the motor that matches these requirements. Find a motor and drive that matches the supply voltage, has a rated speed, continuous torque, and

3 Page 3 continued 8 Easy Steps to Selecting the Right Servo peak torque rating that exceeds the values calculated above. Look at the motor s rotor inertia to find one that satisfies the inertia ratio requirement for the servo drive you are using. Frequently, there may be several motors with similar torque and speed characteristics but different rotor inertias. If there is a motor that is a close match, you are finished. If not, gearing can be applied to match the motor and load more closely. capabilities compared to the other interfaces. Finally, choose any options such as keyed motor shafts, shaft seals, holding brakes for vertical loads, or external braking resistors. Selecting the best servo system for an application is a skill that improves with practice. When in doubt, it s a good idea to verify your results with the manufacturer or distributor. Gearing Servomotors can produce their full rated torque from 0 RPM up to many thousands of RPM. Few machines can take advantage of these speeds without gear reduction. Gear reduction matches the servo to the load in three ways; reducing the speed, increasing the torque, and lowering the inertia ratio. Speed is reduced proportional to the gear ratio, torque is increased proportional to the gear ratio, and most importantly, the inertia ratio is lowered by the square of the gear ratio. Gearbox manufacturers list the inertia of the servo-grade gearboxes, making it easy to include the gearbox inertia into the torque and inertia calculations. Choose a Motor (Round 2) In Round 1 of the selection, most of the motors available likely were capable of far higher speeds than needed. Divide the motor speed by the required speed and round down to get a starting gear ratio. Then divide the required torque by the gear ratio to find the new required torque. This will narrow the choices down to just a couple of motors. Find a motor with an acceptable inertia ratio. If two motors look equal, choose the one with the lower inertia ratio. Repeat this step a couple of times using motors with different rated speeds, as it s possible that more than one good solution exists. Servo Drive and Options Once the servomotor has been selected, choose a servo drive rated for the correct input voltage and with sufficient output current to drive the servo motor. Servo drives can be controlled via several different interface types. These interfaces include pulse-and-direction digital control, analog control, and other servo networks. A servo network provides highspeed control and feedback, reduced wiring, and superior diagnostics Motor Match from Groschopp Simplifies motor and gearmotor selections Presents search results in ranking order of up to ten best fit matches Uses sophisticated algorithms to calculate and match your desired performance specifications with thousands of motor and gearbox designs in the Groschopp website database. Motor Match Now

4 Page 4 What is Horsepower and How is it Calculated? Courtesy Groschopp Inc. How do I calculate my horsepower? This is a common question engineers receive from customers. To answer this question lets first define what horsepower is. Horsepower like any unit of power is simply a rate at which work is being done. Literally, the horsepower unit originates from an experiment which set out to measure the power of a single horse. It was determined that a horse is capable of performing 33,000 ft-lbf of work per min. We will address to this number later in the explanation. First, a few equations to help you calculate your horsepower: Power So Power Work Time Force Dis tan ce Time For electric motors, power or horsepower can be calculated from the torque and speed. For example, if you have a motor rated for 3,000 RPM and 6 in-lbf then the horsepower is calculated below. Horsepower (3,000 6) ,025 63,025 is a constant when using RPM for speed and in-lbf for torque units. 5,252 is another common constant if the speed is in RPM and torque is in ft-lbf. If the units are different then simply make the unit conversion. The derivation of these constants is done using the 33,000 ft-lbf /min = 1 horsepower. Though horsepower units are a derivative of the 33,000 ft-lbf/min, it is not critical to understanding how to calculate motor horsepower for speed and torque. Another common unit of power that motors are rated in is watts. The conversion from watts to horsepower is watts = 1 hp. Groschopp has developed The MOTORTEC STP calculator, a free, downloadable program that provides a fast, easy way to calculate speed, torque, or power. Conversions of the most commonly used units are selectable, automatically generated and can be printed out. Calculate Speed, Torque and Power Calculate Estimated Electrical Current and Losses for Optimum Motor Selection Easily and Accurately Convert Units of Measurement Customizable, Printed Report Function Calculate Operating Costs. To use the STP Calculator tool online or download it visit Convert and Calculate RPM, Kw, Etc. A tool that calculates and converts speed, torque and power values to determine application specifications Electrical current and losses Calculate operating costs Available online and as a free download Download the Free Calculator

5 Page 5 Case Study: Heavy Automotive Enhancing Product Performance and Functionality The Challenge The customer required a high performance, unique design gearmotor assembly for its heavy automotive (bus) shifter. Their vision was a motor with a back up winding inside the armature that could be engaged in the event of a primary winding failure. enhance the performance and reliability of their transmission product. They wanted a motor with a back-up winding inside the armature, so that if the primary winding failed which could ultimately cause the bus to be stuck in one gear (including Park and Reverse) the secondary winding would engage. Their interest started a design cycle that resulted in the prototyping of Groschopp s first 2-commutator armature, and ultimately a proprietary process for creating it. This including engineering the manufacturing processes to still meet cost objectives even with this unique design. The Solution PM 6034 Right Angle Gearmotor Assembly Redundant motor winding for emergency backup Unique dual-commutator design IP 66 rating for harsh environment Custom design aluminum die-cast gear housing to bolt directly on to the transmission Rigorous 25,000 cycle performance testing Customized Right Angle PM Gearmotor with Dual Commutators There was a domino effect to this design, said Kamstra. It required 2 sets of brushes, Dual Commutators The customer knew what they wanted a backup. When they asked why they couldn t have a twocommutator gearmotor we said we d never done it before, and we d never seen it done. but we figured it out - and we did it! Loren Kamstra, Design Engineer Groschopp This customer is a world leader in the design, development and manufacture of a wide range of electronic transmission and drive train control components and systems. Their controls and systems are designed for use in a variety of vehicles with applications that include drive train controls for medium and heavy-duty vehicles. Just like their customers depend on them to enhance the performance and functionality of their products, this customer trusted Groschopp to and the control had to be modified for this redundancy. We also had to engineer a way to mount a strong, non-conductive brush card on to the gear housing. The resulting design was exactly what the customer was looking for, and because it was designed to bolt directly on to the transmission, no design modifications were required on their end.

6 Page 6 Quiet Gearmotors - Engineering Tips Courtesy Groschopp Inc. Using low-noise planetary speed reducers and metallic or plastic gears, sound can be minimized while maintaining a high torque output With a strategic combination of manufacturing techniques and technology choices, quiet operation in motors, gearboxes and gearmotors can be achieved: Using high resolution imaging techniques to characterize the motor commutator Special manufacturing processes can be employed to reduce brush noise Critically evaluate the alignment of speed reducers to the motor. Special mounting hardware is available and noise could result from sub-optimal gear teeth engagement with the motor shaft. Misalignment of the motor and reducer can also degrade operational efficiency. Read more about quiet gearmotor operation in Groschopp s blog post How To: Reduce Noise Output from a Brushed Motor World Class Support Groschopp Inc. is a manufacturer of highly engineered fractional horsepower electric motors and gearmotors for OEM and distribution products. Our technical team is experienced in providing integrated solutions for demanding applications. Groschopp offers a full line of AC, DC, brushless and universal motors along with a full line of right angle worm, planetary, right angle planetary and parallel shaft gearboxes. Visit Groschopp Online

7 Page 7 Gearmotors: Achieving the Perfect Motor & Gearbox Match A how to guide for achieving optimized application performance For electrical and mechanical design engineers in the process of developing applications, choosing a gearmotor can be a tricky and arduous process. Can motors and gearboxes be purchased separately and then matched for an application? Is it better to specify a pre-engineered gearmotor? What are the differences? Focusing on how to choose the right motor/gearbox combination for your OEM application; Engineering Manager, Seth Hulst and Design Engineer, Loren Kamstra explain cost optimization, performance and other critical application considerations. Outlining the basic motor and gearmotor types and the advantages of each choice will help a designer more easily calculate and choose the best gearmotor for the required application. Even if a new gearmotor is not needed immediately many of the ideas presented will help audit a current gearmotor s performance within an application. Learn how to: Interpret ratings and characteristics of various gearbox types as a total picture Select the right motor based on gearbox ratings Identify key application and performance characteristics (beyond speed, torque and life) such as yield strength, operating speed, mounting position, and loading Avoid common mistakes such as selecting a motor that is too large, problems with overheating, and miscalculating yield torque How torque density affects the heat and life of a motor How to select the correct gearbox ratio There are many factors to consider when choosing a gearmotor. Whether selecting a gearbox and motor separately or choosing from pre-engineered gearmotors, understanding the issues of the application is essential. These issues include speed and torque requirements, in addition to mechanical and thermal limitations. Along with gearmotor selection this whitepaper will also illustrate the common mistakes and give readers tips and tools for avoiding them. While purchasing components and mating them or buying a preengineered gearmotor can be successful, if an integrated gearmotor can be found to meet the requirements; it can be a less complex and expensive process. At times, because of the nature of the application, a more extensive review of gearbox and motor specifications is warranted, and components are chosen separately, both to meet the special demands of the job and to insure compatibility of gearbox and motor under those conditions. Download White Paper Learn how thermal and mechanical factors limit gearbox efficiency and performance

8 Page 8 Tough Jobs Case Study: Medical Bed Application case study The Challenge: The gearmotor needed to drive two positionally synchronized arms on a surgical table High torque output, intermittent duty for loads up to 500 lbs Quiet operation for medical/or environment 20 VDC power supply limitation The Solution: Permanent magnet DC motor for high torque and low noise Two planetary speed reducers driven by one common motor shaft Special reducer component configuration and alignment to minimize noise The Details Highly specialized surgical procedures such as spinal, joint replacement and orthopedic trauma surgery require automated operating room tables that can be positioned with extreme accuracy. Just as important as positional accuracy are high torque output and quiet operation, in a compact gearmotor solution that can drive up to 500 lbs. load. When one of the world s leading designers and manufacturers of such innovative medical products was faced with such a challenge, they partnered with Groschopp to tackle this tough job. With this particular surgical table, two hinged arms had to be driven so that their positional alignment and movement would be completely synchronized at all times. The traditional solution of link chains, belts and pulleys has greater margins of operational error, making it more difficult to synchronize the movement and alignment of the two arms, said Engineering Manager, Seth Hulst. A typical solution would be to have one motor for each hinged arm, but we felt that a single motor with two outputs would provide greater accuracy, and be more cost effective. Compounding the design challenge was the 24 volt power supply specification. Groschopp s design team chose to use a 24V permanent magnet DC Gearmotor, which could provide the needed output given the intermittent operation. The motor was customized for high torque, and its one common motor shaft drives two planetary speed reducer outputs. Because the same motor drives both outputs, there is both mechanical and electrical linkage. If two separate motors were used, a more complex, and more expensive, common control scheme would be required to provide this linkage, added Hulst. The high efficiency of the planetary reducers made it possible to achieve the needed torque while staying below the 20A current limit. Once the Dual Planetary DC Gearmotor solution was proved out, the team went into extensive prototyping and testing to reduce noise. We have many medical OEM applications that require minimal noise, and have developed a number of techniques and product customizations to minimize noise, said Ed Tullar, Sales Manager. We analyze all of the potential sources of noise motor, gearbox, gearmotor assembly and then work to minimize noise in each component. I Know My Specs... Now What? Let Groschopp do the heavy lifting. Simply tell us what values you need and we will match them with one of our products Speed (rpm) Torque (in-lb) Horsepower Submit Your Application Specs

9 Page 9 The Basics of Motor Selection A designer s guide to motor types and customization Tailored towards design engineers and OEM s The Basics of Motor Selection is written to assist readers in the motor selection and customization process. Focusing on four motor types (AC Induction, PMDC, Universal and Brushless DC) Groschopp engineers meticulously outline the drawbacks, advantages and characteristics of each motor type. representation of the input and output parameters of a motor. The input electrical power can be in the form of a DC battery, AC line voltage, rectified AC line voltage, or a wide variety of controls. Affected by application and environmental constraints along with the necessary power needed to move a load, the input power will be volts, amps, and frequency. The output power is the motor speed and torque response required to accomplish the task. If you are involved in motor design or selection at some level whether you have some technical and design challenges with current projects and are looking for some new ideas, planning for future designs or you are responsible for purchasing motors, Groschopp outlines 4 basic steps to optimize application design and performance. 4 Basic Criteria to Optimize Design & Performance 1. Motor Basics what do you need to know to get started in selecting the optimal motor for your application? 2. Application Considerations how to identify and define the major characteristics, objectives and priorities of your application? What roles do environment, duty cycle, and electrical / mechanical load play in the successful application of motors? 3. Motor Types a review and comparison of motor types relative to application considerations giving designers a good starting point in making a motor selection. 4. Customization how does customization change motor performance? An overview about how to maximize motor design to meet OEM performance and cost objectives MOTOR BASICS The purpose of a motor, regardless of the application, is to change electrical power to mechanical power in order to provide rotational movement. Every application will have its own distinct parameters for input and output power. The diagram in Figure 1 provides a visual Figure 1: Motor Input and Output Functions APPLICATION CONSIDERATIONS The motor selection process begins with evaluating the application and ensuring the motor chosen will properly match the needs of the application. Though often overlooked by design engineers, the items on the Application Considerations Checklist are critical to OEM motor design and a successful overall system solution. Using the Application Checklist to collect the application data, and then prioritizing it in order of importance will give a designer direction going forward with motor selection and system design. It is important to note that each application will have its own unique performance requirements that need to be evaluated using the checklist. While the items on the Application Checklist may not be the only factors to evaluate, from Groschopp engineer s long-term experience working with OEMs, the checklist covers the majority of application considerations. Download White Paper

10 Page 10 More on Motors by Steve Meyer, Mechatronics Editor Design World Motor requirements fall into two general categories; 1) the load requirement and 2) the environmental requirements. Frequently it is the latter, the environmental requirements, that create problems. Load requirements can usually be dealt with based on a few primary parameters; speed, torque and duty cycle. Speed and torque are pretty obvious. he duty cycle component is more subtle and motor manufacturers incorporate thermodynamic calculations directly into their sizing programs. Basically it boils down to how long the motor is on and how long it is off. This is an attempt to deal with the ultimate limit of performance, the thermal equilibrium of the motor. Once the system reaches it s maximum temperature, there is no more power to be had. Conversely, with low duty cycles it is possible to generate enormous peak power levels as long as there is time to get rid of the heat. This is why motors are often fan cooled. In recent years many suppliers are using forced air cooling and even water cooling to create more power in packages with fixed size. The ultimate limit in electric motors is the insulation system itself. This has lead to increasingly high temperature insulation products like the polyimide resins of recent years. In permanent magnet motors, the high temperature insulation can cause gradual degrading of the magnets which will reduce the motor s output over time. The tricky part of this problem is that you can t tell. There is no inexpensive way to directly measure torque on an axis of a machine to check for torque roll-off. So we design ever more complex control algorithms in the motor control circuit to prevent such overheating from occurring. On the environmental side, things get a lot more complicated. Every major industry has unique requirements that put significant design constraints on motor construction. Down hole Oil & Gas applications require that the motor be immune to extraordinary pressures from water ingress, high temperature operation and a host of issues from physical packaging issues that are problematic at best. Aerospace, semiconductor, automotive, oil and gas and food industry machinery all have requirements that make applying electric motors a challenge for all of the unique requirements that these industries require. The list of special requirements is endless. And, in part, it is this aspect that makes it difficult for manufacturers to maintain volume production and address custom requirements at the same time. It remains a major issue to balance the constraints of motor design and application requirements with cost effectiveness. And it is these unique opportunities that will drive new motor and drive technology in coming years. Go Ahead, Give it a Try Don t take our word for it; try us out as part of your plan for free. Downloadable 3D Solid Models, 2D drawings and connection diagrams on every product page. Get a jump start on your solution.